1. Field of the Invention
The present invention relates to low voltage metal oxide semiconductor field effect transistor (MOSFET) amplifier circuits, and in particular, to low voltage folded MOSFET amplifier circuits for use in ring oscillators.
2. Description of the Related Art
Referring to FIG. 1, ring oscillators are well known in the art and are used in many applications. In this particular example 10, three stages A1, A2, A3 of fully differential amplifiers 12 are serially coupled in a loop in a negative feedback configuration. During normal operation, this ring will bias the differential nodes, i.e., the positive Vip and negative Vin input signal nodes and positive Vop and negative Von output signal nodes, to the metastable point at which all node voltages are mutually equal.
Referring to FIG. 2, a conventional embodiment 12 of the fully differential amplifier circuits 12 used in such a ring oscillator 10 includes P-type MOSFETs P1a, P1b and N-type MOSFETs N1a, N1b interconnected substantially as shown between a tail current source 22 and the lower power supply rail VSS/GND. The tail current source 22, in turn, is coupled between the higher power supply rail VDD and the source electrodes of transistors P1a and P1b. This tail current source 22 provides a tail current I1 which is a product of two, a reference current Iref and the sum of unity and a common mode factor Δ, i.e., I1=2*Iref*(1+Δ). Lower current sources 24a, 24b each sink a current I2 equal to the reference current Iref, while the N-type MOSFETs N1a, N1b, connected as diodes with their drain and gate electrodes mutually connected, each sink a common mode current I3, which is used to maintain proper biasing of the circuit and is equal to the reference current Iref multiplied by the factor Δ, i.e., I3=Δ*Iref.
As is well known in the art, a typical MOSFET threshold voltage VT is 0.4 volt, while a typical output operating voltage, often referred to as the drain-to-source saturation voltage VDSAT, is 0.2 volt. Accordingly, such a conventional amplifier circuit 12 requires a power supply voltage VDD−VSS equal to 2*VT+3*VDSAT, which is equal to 1.4 volt. While such a minimum power supply voltage requirement is relatively low, as transistor dimensions continue to decrease in an effort to reduce overall integrated circuit sizes and increase circuit densities, power supply voltage decrease as well, thereby making a 1.4 volt minimum power supply voltage begin to look relatively high. Accordingly, it would be desirable to have an amplifier circuit for use in a ring oscillator which is capable of operating at a lower minimum power supply voltage.